Transcript Clouds and Thunderstorms

Fire Weather:
Clouds & T-Storms
Physical structure of a cloud
• Minute water droplets
• Ice crystals
• Combination of both
Why are clouds important for fire weather?
Clouds are indicators of…
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Atmospheric moisture
Atmospheric motion
Instability
Warning of weather change
– Precipitation
– Winds
Cirrus Clouds thin, wispy, feathery
Fire weather:
high-altitude moisture and wind direction and speed
warning of warm-front activity
Fair Weather Cumulus Clouds puffy cotton balls floating in the sky
Flat bases and distinct outlines, irregular shapes (“cauliflower”).
Slight vertical growth; cloud tops = limit of the rising air.
Fire weather: warning of convection in surface layer.
Can later develop into towering cumulonimbus clouds.
Altocumulus Clouds parallel bands or rounded masses
Portion of cloud = shaded.
Formed by frontal or orographic lifting.
Fire weather:
May develop into Altocumulus Castellanus clouds…
Altocumulus Castellanus Clouds (“turrets”)
Convection in unstable layer aloft.
Often result of gradual lifting of air in advance of a cold front.
Fire weather: warning of possible thunderstorms later in the day.
Cumulonimbus Clouds - towering high into the atmosphere
- Moist and unstable air = towering cumulus clouds
Fueled by vigorous convective updrafts (> 50 knots, tops > 60,000 ft)
Thunderhead (with anvil)
Fire weather: gusty and high speed surface winds, dust devils,
whirlwinds, turbulence, downdrafts
Lenticular Clouds
• Indicate waves of air flow caused by strong winds blowing across a
range (“mountain waves”).
• Usually appear over the ridge on the lee side of mountains.
• Fire weather: may increase fire activity if air flows descend
Thunderstorms
• Major influence on fire behavior
– Wind patterns
– Lightning (can cause fires anywhere in the
U.S.)
• Three conditions required:
– Unstable air
– Triggering mechanisms (lifting process)
• Orographic, frontal, convergence,
thermal/convection
– Sufficient moisture in air
Group Exercise
• Describe the processes that occur in each of the
three stages of thunderstorm development:
cumulus, mature, dissipating.
• What is the effect on fire behavior in each
stage?
• Note: you will be given a diagram of each stage
to facilitate your discussion and formulation of
your response.
• Each member of the group should be prepared
to be called upon to present the group’s answer
to the class.
3 Stages of a Thunderstorm
1. CUMULUS STAGE
– Lifting of moist air above
condensation level
– Updrafts increase in speed
– Droplets increase in size
– Light downdrafts (settling of air)
– Gentle wind change
– Fire behavior: convection
columns may cause fire to
become more active
T-Storm stages…
2. MATURE STAGE
– Rain falls from cloud base –
drowndrafts
– Updrafts and downdrafts in
different portions of cloud
– Downdrafts strongest at front edge
(>30 mph)
– Convection cell - maximum height
– Anvil top – points in
direction of travel
– Fire behavior:
turbulent, strong wind
(horizontal flow)
T-Storm stages…
3. DISSIPATING STAGE
• No new condensation
to support cloud growth
• Cell changes to all
downdrafts
• Downdrafts dissipate
and surface signs
disappear
Lightning
• Occurs in a T-Storm when
electrical potential builds up
• Movement of particles with
positive and negative charges
• Atmosphere: positive charge
with respect to the Earth
• Cumulonimbus clouds – alters
and intensifies electric fields –
creates a positive charge on
the ground
Lightning Discharge
Cloud-to-ground:
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between negative lower portion of
cloud and positive charge on ground
Cold stroke: intense current, short
duration (345,000 amps)
Hot stroke: lesser current, longer
duration (200 amps)
Hot stokes more likely to start fires
20% lightning bolts in West = hot
strokes
Cloud-to-cloud
discharges
– Between the negative
charge in the lower
cloud and positive
charge in core of cloud.
Thunder
• Compression wave from sudden heating
and expansion of air along path of
lightning discharge
• Reflected from the ground surface = sound
• Light moves faster than sound – see
lightning first
• Approx. 1 mile to the flash for every 5
seconds elapsed time